1. Field of the Invention
The present invention concerns a radio frequency transmitter arrangement and multiple transmitter arrangement for a magnetic resonance system as well as method to determine a setting parameter of such a transmitter arrangement.
2. Description of the Prior Art
Magnetic resonance (MR) tomography is a technique to acquire images of the inside of the body of a living examination subject. An examination subject is exposed to an optimally homogenous static basic magnetic field (designated as a B0 field) that is generated by a basic field magnet of an MR apparatus. During the acquisition of the MR images, rapidly switched gradient fields, generated by gradient coils, are superimposed on this basic magnetic field for spatial coding.
RF energy pulses of a defined field strength are subsequently radiated into the examination subject with radio frequency antennas (RF antennas). The magnetic flux density of these RF pulses is typically designated B1; the pulse-shaped radio frequency field is generally also called the B1 field for short. MR signals are triggered in the examination subject by the RF pulses and are acquired by RF reception antennas.
An example of a combined RF transmission-reception antenna is what is known as a bandpass birdcage resonator as described in German OS 187 02 256. This has sub-antennas (individual rods) with couplings among one another being partially cancelled by suitable technical measures (overlapping, mutual capacitors, transformers). Variable passive power distribution networks and/or a number of separately activatable radio frequency power amplifiers can be used for the optimization of the radiated B1 field with regard to, for example, homogeneity and power loss. The sub-antennas in the prior art are typically used as location-dependent transmitters.
U.S. Pat. No. 6,411,090 describes RF activation of such a birdcage resonator. The signal of an RF source is distributed to the sub-antennas of the birdcage resonator via a power distributor, a phase shifter and a number of amplifiers. The phase shifter effects an equal phase shift of the signal over a number of sub-antennas.
An arrangement to generate radio frequency fields in the examination volume of an MR apparatus is known from European Application 1 279 968. Resonator segments are thereby used that are electromagnetically decoupled from one another by means of capacitors connected in series, such that a separate transmission channel, via which the radio frequency infeed into the appertaining resonator segment ensues, can be associated with each resonator segment. Because the phase and the amplitude of the RF-infeed can be individually predetermined for each resonator segment, the arrangement enables a nearly complete control of the RF field distribution in the examination volume.
German OS 101 48 445 specifies the use of a Butler matrix to receive RF signals by means of the sub-antennas of a birdcage resonator from a reception volume of an MR apparatus. The (sub-antenna) reception signals are combined with one another into a basic combination and into a number of additional combinations. The goal is an optimized signal-to-noise ratio. The improvement is independent of the frequency and the B0 field strength. German OS 101 48 445 additionally explains that the Butler matrix can act as a distribution element for a transmission signal emitted by a transmitter and supplied to the Butler matrix. Upon being fed into a lowermost row of the Butler matrix, a magnetic resonance excitation signal (B1 field) with a substantially location-independent excitation intensity is generated.
A Butler matrix as a special design of a matrix feed system as explained in detail, for example, in “Taschenbuch der Hochfrequenztechnik”, Meinke-Gundlach, 4th edition (1986), page N64.
The MR images of the examination subject are finally created based on received MR signals. The strength of the MR signals is also dependent on, among other things, the strength of the radiated B1 field. Temporal and spatial oscillations in the field strengths of the excited B1 field lead to unintended changes in the received MR signal that can falsify the measurement result. An inhomogeneous spatial distribution of the amplitude of the B1 field, for example, causes an unwanted dependency of the image contrast on the spatial position. This results from a superimposition of the intensity dependency (caused by the inhomogeneous field distribution) with the intensity distribution, which is determined at the respective location by the tissue material, and which contains the actual desired image information.
One reason for the oscillation in the radiated B1 field distribution eddy currents arising in the patient due to the B1 field. This is an unpreventable part of the coupling of the sub-antennas of the mentioned birdcage resonator. Eddy currents occur particularly strongly at wavelengths of the radio frequency field that are in the range of the subject to be examined, i.e. in medical application, for example, at MR frequencies greater than 100 MHz.
If, in addition to the activation of the rods of a birdcage resonator, separate power amplifiers are used, their outputs leak RF power (some of which is reflected back). Due to the nonlinear dependency of the output impedances on the voltage, the output amplitudes can no longer be predicted by a linear superimposition model. Given large signal amplitudes in proximity to the modulation limits, unexpected distortion or even over-voltages can thus be created at power components.
In order to be able to account for such influence of the B1 field distribution, it would be very advantageous if the B1 field could be quantitatively determined and subsequently radiated with corresponding compensation.
Various methods to determine the B1 field are known. As an example, in a technique known as transmitter adjustment the B1 field is determined by the resulting flip angle, using a double-echo RF pulse sequence.